1. Field of the Invention
This invention relates to processing of fibers and thermoplastic polymers into composite materials which have increased adhesion between the fiber and the thermoplastic. More particularly, the present invention relates to the use of plasma discharge treatment of the fibers followed by immediate coating of the fibers with thermoplastic material prior to exposure of the fibers to other reactive substances.
2. Prior Art
Composite materials are structural materials composed of at least two distinct macroscopic components which are mixed together during fabrication to create materials with desired mechanical properties not present in either component alone. The two principal components of such composite materials comprise a matrix phase of thermoplastic or thermosetting resin and a fiber or filler which is embedded within the matrix.
During the last 30 years development of high strength and high stiffness fibers has enabled construction of very light, strong and stiff structural members which have been use in spacecraft and aircraft applications. Common examples of these high strength fibers include fiber glass, carbon graphite and polyaramid fibers. These fibers are typically much stiffer than the matrix and become the primary load bearing elements within the composite material because the embedded fibers or filler carry most of the applied load. Therefore, stability of the fiber within the matrix is essential. If there is not good adhesion between the matrix and the fiber (referred to hereafter as interfacial adhesion), the fiber will slip, shifting the load to the weaker polymer material making up the composite matrix. This may result in composite failure at less than its designated load capacity.
The prior art has generally recognized the importance of increasing interfacial adhesion in development of strong composite materials. This increased adhesion has typically been achieved by (i) roughening the fiber substrate as is disclosed in U.S. Pat. No. 4,664,936, (ii) inducing crystal nucleation at the fiber surface as occurs with polyetherether ketone on carbon fiber (Polym. Eng. Sci. 26: 633, 1986), (iii) modifying the surface energy of the fiber to promote better wetting by the matrix (U.S. Pat. No. 4,072,769), and (iv) formation of covalent chemical bonds between the fiber and matrix. All of these except matrix nucleation involve some kind of physical or chemical modification of the fiber surface. The effectiveness of these techniques will vary, depending upon the selection of matrix materials used in combination with the fiber.
Composite matrix materials are generally divided into two classifications: thermosetting and thermoplastic compositions. Thermosetting materials require chemical reactions to cure them from monomers into rigid, crosslinked polymeric materials. Common examples are epoxy, unsaturated polyester, phenolic, resorcinol-formaldehyde-latex, and urea-formaldehyde resins. Once these resins are cured, the chemical reactions cannot be reversed, and a composite structure cannot be reshaped or reprocessed. Most thermosetting composite materials tend to be brittle and more fragile than thermoplastic composites.
Thermoplastics are linear polymers which are solid at room temperature and can be melted at higher temperatures to facilitate processing into composite materials. They have several advantages over thermosetting resins such as toughness (not brittle), re-useability of scraps, no required curing reactions, indefinite shelf life and lower cost. The major disadvantage of thermoplastic resins is that they are unreactive toward forming chemical bonds with fibers. This lack of chemical reactivity gives thermoplastics good chemical and heat resistance, but poor interfacial adhesion. This poor interfacial adhesion has limited the utility of thermoplastics in high stress applications because reinforcing fibers tend to debond from the thermoplastic matrix. Thus there is a lack of available composite materials which are both tough and have good interfacial adhesion.
Prior art technology for enhancing adhesion between the fiber filler and thermoplastic matrix has been limited to a few specialized polymers which can crystalize or which have a particular surface energy. As was mentioned above, some increased adhesion to thermoplastics has been accomplished by inducing crystalization of the thermoplastic material at the fiber surface. Polyetherether ketone is an example of this type of composition and technique of enhancing adhesion. Some progress has also been made in modifying the wettability of the fiber. Here again, however, these techniques find limited application. What is needed is a useful technology of general application which can be applied to a wide range of thermoplastics for advancing interfacial adhesion.
As has been previously mentioned, improved covalent bonding between the filler and thermosetting resins has been realized. For example, fibers have been subjected to plasma discharge to develop chemical modification on the fiber surface. This chemical modification has been shown to improve wettability of the fiber with the matrix, as well as to create functional groups on the fiber which will react covalently with thermosetting matrix resins.
With respect to improving wettability, a prior art publication by Liston (J. of Adhesion 30: 119, 1989) has shown that treatment of several polymers, glass and fiberglass composites with various gas plasmas increases wettability toward water and epoxy matrix, and increases the bond strength with a thermosetting epoxy. Other studies have used oxygen, nitrogen and argon microwave plasmas on Kevlar (TM) to increase adhesion to a thermosetting triazine matrix material. (J. of Appl. Polym. Sci., 26: 2087, 1981). In an example employing a thermoplastic matrix, Jang et al. used radio frequency in propylene gas to polymerize a thin coating of crosslinked propylene on Kevlar and carbon fibers (Interfaces. in Polym., Ceramics and Metal Matrix Comp., vol. 1 pp. 319, 1988. When subsequently coated with a polypropylene matrix, the resulting composites showed up to 40% increase in interfacial adhesion which is attributed to increased wettability.
There are several technologies which use plasma to place specific chemical groups on fibers which react with epoxide groups to form covalent bonds between the fiber and the thermosetting matrix. Molecular Characterization of Composite Interfaces, discloses the exposure of Kevlar to radio frequency plasma in ammonia and monomethyl amine, placing a primary amine on the fiber which later reacted with an epoxy matrix and increased the composite strength. Similarly, 33rd Int. SAMPE SYMP. used ammonia plasma to place amine groups on polyethylene fiber which greatly increased fiber adhesion in epoxy composites. Carbon fiber and carbon black surfaces have been modified by plasma discharge in fluorocarbon gases and ammonia to place fluorite and nitrogen species on the surface which can improve adhesion to various polymers. J. Mater. Sci., 22: 2937, 1987.
Accordingly, it is generally recognized that a plasma is useful in enhancing the wettability of both thermoplastic and thermosetting resins and in increasing the occurrence of covalent bonding of thermosetting resins at filament interfaces. The utility of plasma discharge with respect to these objectives is believed to arrive because of the generation of surface free radicals on the fiber or filler surface. For example, plasma discharge creates a plasma or ionized gas which is usually produced by the interaction of electromagnetic energy with the gas molecules. This electromagnetic energy may be supplied by direct current, shock waves, lasers, charged particle beams, neutral particle beams, etc. In addition to free electrons, plasmas may contain atomic or molecular ions, free radicals, neutral gas, or other electrically-neutral species.
Such plasma treatment is effective in creating a high density of free radicals on the surface of organic materials either by direct attack of gas-phase electrons, free radicals and ions, or photo decomposition of the surface by the vacuum-ultraviolet light generated in the plasma. These surface free radicals can then react with each other, with species in the plasma phase, or with molecules in the atmosphere such as O.sub.2 once the surface is removed from the plasma environment. Such free radicals can remain at the surface for seconds or even hours if the material is kept in a vacuum. Clark, D.T. Polymer Surfaces. Residual radicals are usually quenched by atmospheric oxygen when the substrate is removed from the plasma reactor. These chemical reactions affect only the top ten nm of the substrate, leaving unchanged the bulk chemistry and the physical and mechanical properties of the material.
Although application of plasma to thermosetting and thermoplastic resins has generated some improvement in wettability, and some enhancement with respect to covalent bond formation with thermosetting materials, general application of this methodology for filler within a thermoplastic composite has not realized a significant level of commercial success. One reason for this nominal progress may be the weakening effect which plasma discharge has demonstrated with respect to fiber or filler. In many instances, as observed by the present inventors, the subjection of the fiber or filler to a plasma reduces the strength of the fiber or decreases the fiber diameter. In addition, the degree of improvement in interfacial adhesion with respect to thermoplastic materials has not been as promising as the application of plasma discharge to filler material used within thermosetting resins. In all of these prior art technologies, it is significant to note that the fiber or film was treated with plasma and subsequently removed from the plasma reactor before being coated with matrix material. This latter step of applying the matrix coating to the filler appears to have been accomplished in all instances external to the reactor.